A known technique for improving the mechanical and thermal properties of the bitumen used as a binder for road-making is to incorporate elastomers therein.
In this application, as in many others, it is common practice to use vulcanized elastomers, i.e. elastomers in which the macromolecular chains are connected to one another by sulfur atoms in order to form a three-dimensional lattice. When applied to binders for road-making, vulcanization stabilizes the dispersion of the elastomer in the bitumen and makes it possible to reduce the fraction of elastomer which is required for obtaining the desired improvement in properties.
Whatever the application, an elastomer is vulcanized by making it react with a vulcanization agent. The most conventional vulcanization agent is sulfur itself, however various sulfur compounds, in particular organic compounds of sulfur, has been proposed as vulcanization agents. Vulcanization additives such as accelerators, retarders, and anti-oxidants are commonly used in conjunction with a vulcanization agent.
It has been observed that sulfur is poorly wetted in a molten mass of bitumen and elastomer, thereby giving rise to poor dispersion and to a violent reaction which gives off a large amount of heat, thereby degrading the elastomer. Further, it is practically impossible to predict the degree of vulcanization which will be achieved using a given quantity of sulfur, and as a result the mixture may turn out to be insuficiently vulcanized, or on the contrary it may constitute an unusable gelled mass. Finally, it can happen that after coming to an initial halt, the vulcanization reaction starts up again much later when the temperature is raised, e.g. after the binder has been spread on a road, and this can give rise to undesirable changes in the properties of the binder.
The above drawbacks also exist, although sometimes to a lesser extent, with other prior art vulcanization agents and in other applications of vulcanized elastomers.
An essential object of the present invention is to enable the vulcanization agents to be thoroughly dispersed throughout the mass to be vulcanized and to cause the vulcanization reaction to take place evenly until it reaches a predetermined final degree of vulcanization.
It has also been observed that sulfur and other prior art vulcanization agents form polysulfide bridges between the elastomer chains, i.e. bridges constituted by a plurality of sulfur atoms in series, and these bridges are relatively fragile and consequently they run the risk of breaking during the vulcanization reaction, or later on. Another object of the invention is to propose a vulcanization agent which gives rise to stronger bridges than those obtained using elemental sulfur.